This invention relates generally to semiconductor lasers and light-emitting diodes and more particularly to a nonlasing superluminescent LED suitable for applications requiring a low internal reflectivity source.
There is a substantial need for fast and bright sources for optical communications and other purposes. At wavelengths of 1.3 and 1.55 microns, all sources suffer from Auger recombination, an essentially nonradiative process which competes with the active radiative mechanism, thus reducing the light output. In a normal light-emitting diode (LED), the radiative mechanism is spontaneous bimolecular radiative recombination. In a laser, the radiative mechanism is stimulated emission. Both the Auger recombination and the bimolecular recombination rates increase with increasing carrier density, but the Auger rate increases more rapidly. Because Auger recombination is so strong, the carrier density must be kept low for high efficiency, but then the decay rate is so slow that the LED is not very fast. Alternatively, the carrier density can be driven higher so that the decay rate is fast, but then Auger recombination dominates and the LED is very dim.
Stimulated emission is a more rapid process than bimolecular recombination at the lower carrier densities mentioned above, thus competing more effectively with Auger recombination. Quantum well solid state lasers with fast switching of large optical signals are described in K. Berthold, et. al., Voltage-controlled Q switching of InGaAs/InP single quantum well lasers, Appl. Phys. Lett. 55(19), pp. 1940-42, Nov. 6, 1989, and D. R. Dykaar, et. al., Large-signal picosecond response of InGaAs/InP quantum well lasers with an intracavity loss modulator, Appl. Phys. Lett. 56(17), pp. 1629-31, Apr. 23, 1990. These devices have a quantum well structure having a cavity length L.sub.c of 300 microns divided into long gain regions (L.sub.g =268 micron) and a short intracavity absorption region (L.sub.A =2-20 microns) that can be biased with 6 micron separations (total L.sub..alpha. =14-32 microns) . With a suitable combination of forward and reverse biases applied respectively to the gain and absorption regions, the laser diode can be made to operate in either normally-on or normally-off states, and can be actively Q-switched by voltage control to generate short optical pulses. These devices have been adapted to digital optical switching by A. F. J. Levi et. al., Multielectrode quantum well laser for digital switching, Appl. Phys. Lett. 56(12), pp. 1095-97, Mar. 19, 1990.
The output of a laser may suffer, however, from problems of catastrophic degradation. Importantly, the narrow bandwidth of a laser optical output is unsuited for many applications such as optical time domain and optical coherence domain reflectometry (also called precision reflectometry). For precision reflectometry, it is preferable to have a source that provides a high power output over a broader bandwidth than a laser provides. Coherence domain reflectometry also requires a low reflectivity light source, preferably having internal reflected signal amplitudes 70 dB or more below the output signal level.
Superluminescent LEDs (SLEDs) utilize stimulated emission as their primary radiative mechanism, but do not exceed the threshold for oscillation. They do not form hot spots and do not catastrophically degrade. SLEDs made of bulk material tend to lase when the temperature is lowered, however, confining the operating specifications to an impractically narrow range of temperature. To prevent lasing at long wavelengths, pumping must be further decreased, at the cost of output power.
Because of this problem (and perhaps others), researchers have concentrated on fabricating SLEDs using low mirror losses to prevent regenerative oscillation. Antireflection coatings are difficult to manufacture, however, and these devices are very susceptible to lasing as a long cavity laser if light is reflected back into them from an external surface. Another approach, employed in a commercially-available LED offered by Laser Diode Inc. of Edison, N. J. (LDI) for use in reflectometry, uses a 178 micron active gain region and a 533 micron passive absorber, both formed in bulk material. This device has an internal reflection signal power of about 40 dB below the main output signal even with 3% antireflection coatings on both facets and low internal reflectance. While the device is usable and better than other available sources, this level of reflections is still much higher than optimal for precision reflectometry.
Accordingly, a need remains for faster, broadband, and bright sources for optical communications, reflectometry and other uses.